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HREF="node66.html">Voice tags</A>
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HREF="node58.html">Automation and voice management</A>
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HREF="node64.html">Polyphony</A>
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<H1><A ID="SECTION00850000000000000000"></A>
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<A ID="sect4.voicealloc"></A>
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<BR>
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Voice allocation
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</H1>
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<P>
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It is frequently desirable to automate the selection of voices to associate
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with individual
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<A ID="4677"></A><I>tasks</I>
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(such as notes to play). For example, a musician playing
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at a keyboard can't practically choose which voice should go with each note
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played. To automate voice selection we need a voice allocation algorithm, to
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be used as shown in Figure <A HREF="#fig04.09">4.9</A>.
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<P>
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Armed with a suitable voice allocation algorithm, the control source need not
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concern itself with the detail of which voice is taking care of which task;
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algorithmic note generators and sequencers frequently rely on this. On the
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other hand, musical writing for ensembles frequently specifies explicitly
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which instrument plays which note, so that the notes will connect to each other
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end-to-end in a desirable way.
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<P>
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<DIV ALIGN="CENTER"><A ID="fig04.09"></A><A ID="4682"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 4.9:</STRONG>
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Polyphonic voice allocation.</CAPTION>
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<TR><TD><IMG
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WIDTH="166" HEIGHT="388" BORDER="0"
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SRC="img372.png"
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ALT="\begin{figure}\psfig{file=figs/fig04.09.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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One simple voice allocation algorithm works as shown in Figure <A HREF="#fig04.10">4.10</A>. Here we
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suppose that the voice bank has only two voices, and we try to allocate voices
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for the tasks <IMG
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WIDTH="11" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
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SRC="img4.png"
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ALT="$a$">, <IMG
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WIDTH="10" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img21.png"
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ALT="$b$">, <IMG
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WIDTH="10" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
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SRC="img293.png"
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ALT="$c$">, and <IMG
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WIDTH="11" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img28.png"
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ALT="$d$">. Things go smoothly until task <IMG
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WIDTH="11" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img28.png"
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ALT="$d$">
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comes along, but then we see no free voices (they are taken up by <IMG
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WIDTH="10" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img21.png"
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ALT="$b$"> and <IMG
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WIDTH="10" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
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SRC="img293.png"
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ALT="$c$">).
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We could now elect either to drop task <IMG
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WIDTH="11" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img28.png"
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ALT="$d$">, or else to steal the voice of
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either task <IMG
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WIDTH="10" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img21.png"
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ALT="$b$"> or <IMG
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WIDTH="10" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
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SRC="img293.png"
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ALT="$c$">. In practice the best choice is usually to steal one.
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In this particular example, we chose to steal the voice of the oldest task,
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<IMG
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WIDTH="10" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img21.png"
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ALT="$b$">.
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<P>
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<DIV ALIGN="CENTER"><A ID="fig04.10"></A><A ID="4688"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 4.10:</STRONG>
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A polyphonic voice allocation algorithm, showing voice stealing.</CAPTION>
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<TR><TD><IMG
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WIDTH="371" HEIGHT="218" BORDER="0"
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SRC="img373.png"
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ALT="\begin{figure}\psfig{file=figs/fig04.10.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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If we happen to know the length of the tasks <IMG
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WIDTH="10" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img21.png"
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ALT="$b$"> and <IMG
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WIDTH="10" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
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SRC="img293.png"
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ALT="$c$"> at the
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outset of task <IMG
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WIDTH="11" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img28.png"
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ALT="$d$">, we may be able to make a better choice of which voice
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to steal. In this example it might have been better to steal from <IMG
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WIDTH="10" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
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SRC="img293.png"
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ALT="$c$">, so
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that <IMG
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WIDTH="11" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img28.png"
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ALT="$d$"> and <IMG
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WIDTH="10" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img21.png"
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ALT="$b$"> would be playing together at the end and not <IMG
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WIDTH="11" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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SRC="img28.png"
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ALT="$d$"> alone.
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In some situations this information will be available when the choice must be
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made, and in some (live keyboard input, for example) it will not.
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HREF="node66.html">Voice tags</A>
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